The present invention relates to an SSB (single side band) mode transceiver and, more particularly, to such a transceiver which can be automatically tuned to an incoming signal frequency.
The allowance of frequency deviation among wireless telecommunication stations is a very significant matter. For example, the frequency deviation which is demanded for a Class D station in citizen band is within .+-.0.0005%. This value is equal to .+-.1350 Hz on the CB frequency. In typical AM mode operation this allowance is not very important. However, in SSB mode this allowance should be held within a range of 50 to 100 Hz so that the telecommunication can be accomplished with a satisfactory readability. In order to accomplish SSB mode operation by a Class D station it is often necessary to tune the transceiver to the same frequency as the other station by means of a manual receiver frequency correction means often referred to as clarifier or fine tuner. It goes without saying that a channel selector must also be used.
A conventional SSB transceiver will be described with reference to FIGS. 1 and 2 which are schematic block diagrams respectively showing transmitter and receiver units of a conventional transceiver. When the transceiver is in the transmission mode, the audio signal from a microphone (MIC) is amplified by means of a speech amplifier 6. The amplified signal is fed to a balanced modulator in which it is converted into a carrier suppressed double side band (DSB) signal by modulating a carrier signal from a carrier oscillator 8. The DSB signal is then converted into a single side band signal after passing a crystal filter 4. The SSB signal is applied to a transmitter mixer 3 in which it is converted into a transmitting signal by being mixed with a signal from a local oscillator 7 comprising a phase locked loop type frequency synthesizer. The transmitting signal is power amplified through a driver 2 and a power amplifier 1 so that it may be transmitted from an antenna (ANT) at a desired power. The aforementioned carrier oscillator 8 is comprised of a quartz crystal oscillator which oscillates at a single frequency. The oscillator is also commonly used for generating a demodulating signal for the receiver unit. The PLL synthesizer 7 is adapted to synthesize desired frequencies by means of one or two crystal elements and the selection of a frequency is accomplished in a digital manner by means of a channel selection switch (not shown). When the transceiver is in a transmission mode, a constant biasing voltage is applied to the PLL synthesizer 7 through constant resistors, since the output frequency from the synthesizer 7 may be changed only when the transceiver is in the receiving mode.
In the receiving mode an incoming signal is amplified by radio frequency amplifier 9 and applied to a mixer 10, in which the amplified incoming signal is converted into an intermediate frequency (IF) by being mixed with a signal from the PLL synthesizer 7. The intermediate frequency signal is passed by a crystal filter 4 so that only a desired frequency band of the signal may be selected. The filtered signal is amplified and fed to a balanced detector 12 in which the signal is demodulated into an audio signal by being mixed with a signal from the carrier oscillator 8. The demodulated signal is amplified to a suitable level to drive a loud speaker (SP) by means of an audio amplifier 13. The PLL synthesizer 7 is provided with a variable resistor forming a clarifier or fine tuner for making the receiver frequecy equal to the frequency of the other transmitting station when the transceiver is in the receiving mode. If the receiver frequency is different from the transmitting frequency of the other station, the readability of the demodulated signal is low. Normally, the biasing voltage which is applied to a voltage controlled oscillator of the PLL synthesizer 7 is adjusted by manually operating the variable resistor so that the receiver frequency is made equal to the incoming frequency. This operation is well known as zero-beat operation.
The frequency shift due to mechanical and environmental changes is on the order of 1000 Hz in conventional transceivers. Thus, in SSB operation a receiver frequency adjusting device is essential. However, this device makes transceiver operation difficult and driving unsafe if a car should be equipped with such a transceiver. Furthermore, owing to this troublesome operation, transceiver operators prefer typical AM to SSB modes so that interference problems will be minimal.
This in order to accomplish high readability SSB telecommunication without manually operating a frequency adjusting device such as a clarifier, the permissible frequency difference between stations may be in the order of .+-.25 to 50 Hz. This value is equal to 1/27-1/54 of frequency deviation in conventional transceivers. This requirement cannot practically be met by a typical CB transceiver, since the crystal oscillator is activated in a constant temperature bath.